1. Field of the Invention
The present invention relates to a photographic film cassette, more particularly to a self-advancing photographic film cassette in which rotation of a spool causes photographic film to advance to the outside of a cassette shell.
2. Description Relative to the Prior Art
A known film cassette includes photographic film positioned so that the film leader does not protrude from the cassette shell prior to loading the cassette in a camera. Such a film cassette is easily loaded into a camera. Simple feeding mechanisms are used with the latter type of film cassette and include a construction which rotates the spool to unwind the film, thereby causing the leads to move through a film passageway and exit the cassette. A film cassette disclosed in U.S. Pat. No. 4,834,306 has a spool provided with rotatable flanges on both lateral sides of the film wound in a roll. Each flange consists of a disk and a circumferential lip. Lips are formed on the periphery of the disks to project in the direction along the length of the cassette. Spreading projections are formed on the inside of the cassette shell in positions where the film is directed to the film passageway for deforming the flanges outwardly to widen the interval between the flanges. The spreading projections continuously spread the flanges in the positions close to the film passageway so as to release the outermost turn of the film from contact with the lips. Therefore, the film leader is advanced to exit the cassette through the film passageway by rotating the spool in the direction to unwind the film.
However, the film cassette described in the above U.S. Pat. has a disadvantage in that the flanges cannot recover their initial shape, because the continuous deformation of the flanges by means of the spreading projections gives rise with time to a creep of the flanges, though the deforming load is considerably small. If the flanges become permanently deformed, the lips of the flanges might not cover the edges of the outermost turns of the film, and this may cause the film to be so loosened that film leader will not advance when the spool is rotated. There is also a problem of a large friction between the contact surfaces of the spreading projections and the flanges spread by the spreading projections. A large load applied to the spool causes necessity of a camera having a film feeding mechanism capable of providing a large torque for rotating the spool, of which the manufacturing cost might be enlarged.